Embodiments of the inventive concept relate generally to semiconductor memory devices. More particularly, embodiments of the inventive concept relate to variable resistance memory devices and related methods of operation.
Semiconductor memory devices can be roughly divided into two categories according to whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power.
In recent years, there has been a steady increase in the demand for nonvolatile memory devices capable of high speed operation and high storage capacity. This demand has been addressed to a certain extent by the widespread adoption of flash memory. However, researchers continue to explore additional types of nonvolatile memory devices with the potential for improved performance and data storage capacity. Examples of these additional types of nonvolatile memory devices include ferroelectric random access memory (FRAM), magnetoresistive random access memory (MRAM), phase change random access memory (PRAM), and resistive random access memory (RRAM).
RRAM is viewed as having the potential for relatively high speed, high storage capacity, and low power consumption. In an RRAM, data is stored using a variable resistance material layer having resistance that can be changed according to a polarity or magnitude of an applied electrical pulse. In some RRAMs, the variable resistance material layer is formed of a colossal magnetoresistive material layer having a perovskite structure or a metal oxide layer in which a conductive filament is modified by an electric pulse. In general, any memory using a variable resistance material layer can be referred to as a variable resistance memory.
Variable resistance memory devices can be classified into unipolar variable resistance devices and bipolar variable resistance devices. In a unipolar variable resistance device, a set pulse for performing a set programming operation and a reset pulse for performing a reset programming operation have the same polarity. In a bipolar variable resistance device, a set pulse for performing a set programming operation and a reset pulse for performing a reset programming operation have a different polarity.
The characteristics of different memory cells of a variable resistance memory device can vary according to a number of factors, such as variances in manufacturing processes, different usage patterns, and so on. Unfortunately, these different characteristics can lead to inconsistent operation and errors. For instance, they can cause memory cells to represent the same target state with different resistance levels, which can lead to read errors.